Disclosed herein are fiber reinforced thermoplastic compositions comprising at least one of a polyimide, polysulfone, polycarbonate, polyestercarbonate or polyarylate. The thermoplastic compositions contain uniformly dispersed fibers that provide formed parts with improved strength and modulus compared to the compositions with no fiber. The compositions further comprise a sulfonate salt that improves ignition resistance and has a surprisingly beneficial effect on increasing melt flow.
Glass and mineral fibers are commonly used in compositions with engineering thermoplastics to improve strength and modulus. However, addition of these fibers has such drawbacks as increase in weight, loss of elongation, appearance of anisotropic properties and loss of melt flow in the resulting compositions. The loss of melt flow is especially troublesome in amorphous thermoplastic resins with high glass transition temperature (Tg) (i.e. those with Tg greater than 145° C.). High Tg amorphous thermoplastic resins with useful mechanical properties are high molecular weight and generally are more difficult to melt process than higher flowing crystalline resins. In fiber-containing compositions of high Tg resins the melt flow is further reduced over that of the base resins not containing fiber. In many instances the only resort to mold parts from such compositions is to increase temperature in molding equipment. However, the very high temperatures encountered (typically 300-400° C.) can often result in thermal degradation of the thermoplastic resin leading to the loss of properties and/or the generation of volatile products producing unacceptable molded parts. Thus there exists a need to improve the melt flow and processability of fiber-filled high Tg amorphous thermoplastic compositions.
In addition some high Tg thermoplastic resins are more easily ignited than others rendering them unfit for some applications where the ignition and burning of fiber filled plastic parts may be a concern. This is true of some blends of polycarbonate (PC) with polyetherimide (PEI) as described in U.S. Pat. No. 4,548,997 and related blends comprising PEI and polyarylate resins that are disclosed in U.S. Pat. Nos. 4,908,418 and 4,908,419.
Efforts to improve the flame retardancy of PC-PEI blends with brominated polystyrene resin are disclosed in U.S. Pat. No. 4,629,759. Use of brominated flame retardants often causes problems due to the decomposition of the brominated compound at high melt processing temperature of these blends giving acidic species that can corrode molds and machinery. In addition halogenated flame retardants are becoming increasing unpopular in some areas due to potential environmental concerns.
Several other patents, for instance U.S. Pat. Nos. 5,051,483 and 6,011,122, describe the addition of silicone polyetherimide copolymers to improve flame retardant (FR) properties of PC-PEI compositions. While effective, use of an additional ingredient such as a silicone copolymer adds expense and complexity to the manufacture of said composition.
Another issue of blends such as those of PC with PEI is their poor melt processing characteristics when combined in the ratio of about 30-70 to 70-30. These blends are very difficult to compound on an extruder and show surging and excessive die swell with poor melt elasticity. The blend extrudate is very hard to strand and cut into pellets. This limits the use of such blends. Typically the addition of even a small amount of fiber glass removes the melt flow instability. The glass fibers also improve strength and modulus of the blend. However the melt flow of the blend, while more uniform, is reduced. This reduction in melt flow makes it harder to mold parts. Therefore, while there has been significant work in this area several problems still exist with regard to preparing flame and ignition resistant fiber filled high Tg amorphous thermoplastic compositions.